A favorite theme can give a birthday its own world. For this mermaid table, seafoam, lilac and pink carry the under-the-sea feeling across a cake, cakesicles and dipped sweets. Shell details help connect the different treats without making them all the same.
A larger cake gives the table a focal point, while the smaller sweets leave room for the theme to show up in other ways. A familiar color on one treat and a shell shape on another can be enough to make the collection feel connected.
The most useful starting point is often what the person celebrating loves. That might be a story, a color or a theme they return to again and again. You can begin there before deciding exactly which sweets belong on the table.
For a themed dessert mix, keep the practical details close to the creative ones. A guest count, a date and a few favorite treats can help turn a broad idea into something you can discuss. There is no need to have every choice made at the start.